OK, well I have asked the question of a seller of one of these things......
You have to be aware with a reflector that although the objective is 76mm, you lose some light because of the reflector on the front. Not sure of the exact amount, but it may be around 25%.
...
What is the diameter of the centre obstruction (secondary reflector attached to the rear of the objective lens)?
I have just received a reply which states it is 18mm.
So - pulling out the old calculator again I think I can figure out what it's effective equivalent obstruction free diameter would be...
sqrt(78^2-18^2) = 75.8946638
So in actual fact the 78mm lens is equivalent to a 75.9mm lens. If I have done my sums correctly, that is 2.7% drop in diameter and a 5.3% drop in area.
OK, a 76mm lens is not an 80mm lens but I am also looking at the compactness. It comes with a 26-78x zoom lens - judging by the comments received, the lower end of the magnification will be just about right and the mid-higher end should be fine for basic stargazing. It also comes with a fixed 19.5x lens with wider fov. I like the fact it takes any 1.25" lens too.
I am currently watching a couple of these in auction, hopefully I will win one of them. I'll let you know if I do, however I won't be able to give an accurate review as I am not experienced enough. i will however let you know whether I like it or not.
p.s. can someone delete post 16 It has been duplicated and superceded with post 17, but I can no longer edit it.
many thanks